1. Field of the Invention
The present invention relates to improved cementing compositions and their use in cementing operations, particularly for cementing wells penetrating subterranean formations.
In particular, the present invention relates to additives to cementing compositions employed in the cementing of oil and gas wells, for the purpose of reducing fluid loss from the cementing composition to the surrounding subterranean formation which the well has penetrated.
More particularly, the present invention relates to polyvinylpyrrolidone-polyacrylamide triblock polymers as fluid loss additives to aqueous hydraulic cement slurries and the use of such slurries in well cementing operations, especially under conditions of high temperature and with brine as well as fresh water.
2. The Prior Art
Polymer compositions have long been used as additives to cementing compositions for the purpose of reducing water loss. For example, U.S. Pat. No. 2,614,998 discloses the use of water-soluble alkali salts of partially hydrolyzed polyacrylamide and polyacrylic acid for reducing the filtration of water from cement slurries. Polyvinylpyrrolidone is known as a fluid loss additive. Thus, U.S. Pat. No. 3,359,225 describes the use of polyvinylpyrrolidone to prevent the separation of free water from a cement slurry to which it is added and U.S. Pat. No. 3,511,313 discloses a method for sealing void spaces in a geological formation with a liquid slurry of water-swellable poly-N-vinyl pyrrolidone in an organo solvent therefor. It is also known in the cementing art to employ copolymers of polyvinylpyrrolidone as fluid loss additives to cement. U.S. Pat. No. 3,140,269 discloses inhibiting the fluid loss in an hydraulic cement composition by adding thereto a polymer prepared by copolymerizing maleic anhydride and N-vinylpyrrolidone. And U.S. Pat. No. 3,421,584 discloses a method of inhibiting fluid flow in terranean formations by emplacing in the formation an autonomously gelable composition containing a polymer prepared by polymerizing acrylamide and vinylpyrrolidone. Finally, U.S. Pat. No. 3,415,319 discloses an aqueous acid fluid for use in acidizing and hydraulic fracturing of oil- or gas-bearing formations which contains a copolymer of acrylamide and vinylpyrrolidone added for the purpose of controlling undesirable loss of fluid to a formation being treated. It has been found, however, that the polyvinylpyrrolidonepolyacrylamide triblock polymers of the present invention exhibit fluid loss reducing properties notably superior to those possessed either by vinylpyrrolidone homopolymer or by a random copolymer of vinylpyrrolidone and acrylamide.